Convoys, coming mainly from North America and predominantly going to the United Kingdom and the Soviet Union, were protected for the most part by the British and Canadian navies and air forces.

This front ended up being highly significant for the German war effort: Germany spent more money on producing naval vessels than it did every type of ground vehicle combined, including tanks.

[27] These regulations did not prohibit arming merchantmen,[28] but doing so, or having them report contact with submarines (or raiders), made them de facto naval auxiliaries and removed the protection of the cruiser rules.

In contrast with Hitler and Erich Raeder, the chief of the German Navy, he judged that war with the UK was inevitable and that a large surface fleet was not needed, but that U-boats could defeat the British.

Many German warships were already at sea when war was declared in September 1939, including most of the available U-boats and the "pocket battleships" Deutschland and Admiral Graf Spee which had sortied into the Atlantic in August.

The Royal Navy quickly introduced a convoy system for the protection of trade that gradually extended out from the British Isles, eventually reaching as far as Panama, Bombay and Singapore.

German success in sinking Courageous was surpassed a month later when Günther Prien in U-47 penetrated the British base at Scapa Flow and sank the old battleship HMS Royal Oak at anchor,[g] immediately becoming a hero in Germany.

The British and French formed hunting groups including three battlecruisers, three aircraft carriers, and 15 cruisers to seek the raider and her sister Deutschland, which was operating in the North Atlantic.

It was in these circumstances that Winston Churchill, who had become Prime Minister on 10 May 1940, first wrote to President Franklin Roosevelt to request the loan of fifty obsolescent US Navy destroyers.

The Italian submarines had been designed to operate in a different way than U-boats, and they had flaws that needed to be corrected (for example huge conning towers, slow speed when surfaced, lack of modern torpedo fire control), which meant that they were ill-suited for convoy attacks, and performed better when hunting down isolated merchantmen on distant seas, taking advantage of their superior range and living standards.

Only the sacrifice of the escorting armed merchant cruiser HMS Jervis Bay (whose commander, Edward Fegen, was awarded a posthumous Victoria Cross) and failing light allowed the other merchantmen to escape.

Many of these ships became part of the huge expansion of the Royal Canadian Navy, which grew from a handful of destroyers at the outbreak of war to take an increasing share of convoy escort duty.

To this end, the Admiralty asked the Royal Canadian Navy on 23 May, to assume the responsibility for protecting convoys in the western zone and to establish the base for its escort force at St. John's, Newfoundland.

On 21 May, SS Robin Moor, an American vessel carrying no military supplies, was sunk by U-69 750 nautical miles (1,390 km) west of Freetown, Sierra Leone.

[75] In early 1941, the Royal Navy made a concerted effort to assist the codebreakers, and on 9 May crew members of the destroyer Bulldog boarded U-110 and recovered cryptologic material, including bigram tables and current Enigma keys.

Admiral Ernest King, Commander-in-Chief United States Fleet (Cominch), who disliked the British, initially rejected Royal Navy calls for a coastal blackout or convoy system.

The US did not have enough ships to cover all the gaps; the U-boats continued to operate freely during the Battle of the Caribbean and throughout the Gulf of Mexico (where they effectively closed several US ports) until July, when the British-loaned escorts began arriving.

One tactic introduced by Captain John Walker was the "hold-down", where a group of ships would patrol over a submerged U-boat until its air ran out and it was forced to the surface; this might take two or three days.

U-boats were relatively safe from aircraft at night for two reasons: 1) radar then in use could not detect them at less than 1 mile (1.6 km); 2) flares deployed to illuminate any attack gave adequate warning for evasive manoeuvres.

The common practice of surfacing at night to recharge batteries and refresh air was mostly abandoned as it was safer to perform these tasks during daylight hours when enemy planes could be spotted.

It also caused problems for the Germans, as it sometimes detected stray radar emissions from distant ships or planes, leading U-boats to submerge when they were not in danger, preventing them from recharging batteries or using their surfaced speed.

Günter Hessler, Admiral Dönitz's son-in-law and first staff officer at U-boat Command, said: "We had reached a stage when it took one or two days to decrypt the British radio messages.

Seventy-five long range aircraft equipped with the new centrimetric ASV Mark III radar with PPI display patrolled regions in the Bay of Biscay with known concentrations (through enigma decrypts) of U-boats in transit.

[100] At the May 1943 Trident conference, Admiral King requested General Henry H. Arnold to send a squadron of ASW-configured B-24s to Newfoundland to strengthen the air escort of North Atlantic convoys.

[76] Dönitz's aim in this tonnage war was to sink Allied ships faster than they could be replaced; as losses fell and production rose, particularly in the United States, this became impossible.

Despite U-boat operations in the region (centred in the Atlantic Narrows between Brazil and West Africa) beginning late 1940, only in the following year did these start to raise serious concern in Washington.

U-320 was the last U-boat sunk in action, by an RAF Catalina; while the Norwegian minesweeper NYMS 382 and the freighters Sneland I and Avondale Park were torpedoed in separate incidents, hours before the German surrender.

The ordinary sailors had no uniform and when on leave in Britain they sometimes suffered taunts and abuse from civilians who thought the crewmen were shirking their duty to enlist in the armed forces.

These were "over-pessimistic threat assessments", Blair concludes: "At no time did the German U-boat force ever come close to winning the Battle of the Atlantic or bringing on the collapse of Great Britain".

The development of the improved radar by the Allies began in 1940, before the United States entered the war, when Henry Tizard and A. V. Hill won permission to share British secret research with the Americans, including bringing them a cavity magnetron, which generates the needed high-frequency radio waves.